Multilevel power converters have drawn a lot of attention recently. The modulation technique used in multilevel converters must have high efficiency, reduced passive filter cost, and fast transient response under different dynamic conditions. High efficiency is a critical metric for multilevel converters. Because low switching frequencies lead to low switching power losses, low switching frequency modulation techniques such as selective harmonic elimination-PWM (SHE-PWM), selective harmonic mitigation-PWM (SHM-PWM), and selective harmonic current mitigation-PWM (SHCM-PWM) are promising to increase converter efficiencies. In conventional SHE-PWM or SHM-PWM techniques, only the low order harmonics are eliminated or mitigated to meet voltage harmonic limits. Hence, the conventional SHE-PWM and SHM-PWM techniques cannot ensure that current harmonic limits are met, and these limits are more important than the voltage harmonic limits for the grid tied converters. In addition, the grid voltage harmonics can lead to unmitigated current harmonics for SHE-PWM and SHM-PWM techniques, but this information is not included in the equations of these modulation techniques.
These two problems can be considered by introducing a SHCM-PWM technique that can meet the current harmonic limits of IEEE-519 by including the effects of the grid voltage harmonics in the optimization process. In this technique, the coupling inductance between the converter and the grid can be significantly reduced in comparison to SHE-PWM and SHM-PWM techniques. Moreover, a higher number of current harmonics than SHE-PWM and SHM-PWM techniques can be mitigated with the same number of switching transitions. In He et al., based on the dynamic equations of the grid-tied converters, a high performance dynamic response can be achieved for a four-quadrant grid-tied converter. In addition, an indirect controller is used to change the active and reactive currents four times in each fundamental cycle. The modulation technique used in He et al. is phase-shift PWM (PSPWM), which uses a high switching frequency to control low order harmonics. It is important to note that the SHCM-PWM technique could not be used with the indirect controller technique to obtain high dynamic performance. Because SHCM-PWM is an offline modulation technique and the switching angles are calculated and stored in look-up tables, it needs to use fast Fourier transform (FFT), which results in time delays, to apply switching angles to the converters. In addition, the number of switching transitions is very low in SHCM-PWM, so it results in high ripple currents. As a result, it can cause intrinsic weak dynamic performance. When active or reactive power are controlled with SHCM-PWM in four-quadrant converters, because the switching angles need one fundamental cycle to get updated, a DC offset remains on the injected currents for several cycles under dynamic conditions.
A new selective harmonic mitigation-pulse amplitude modulation (SHM-PAM) was proposed to eliminate the triplet harmonics of the CHB converter by controlling the switching angles and the DC-link voltages of cells of the CHB. Also, low-order non-triplet harmonics of the CHB voltage are controlled to meet the power quality voltage requirements. However, this technique needs to change all DC-link voltages of the CHB converter for different modulation indices which can increase the complexity and the cost of the converter.
Recently, a fault-tolerant asymmetric selective harmonic elimination-PWM (asymmetric SHE-PWM) technique for the CHB inverter was proposed in to generate a balanced AC voltage with the three-phase CHB converter when one of the cells has a fault. A real-time selective harmonic elimination technique is also proposed in to find the solutions of switching angles of the low-frequency modulation technique in real-time. An indirect controller was proposed for having a transient-free dynamic response when the active and reactive current of the grid-tied converter is changed twice in a fundamental period. To reach this goal a high switching frequency modulation technique (PS-PWM) was used to change the AC voltage of a grid-tied converter. So similar to using the PS-PWM technique in the transient period, the active and reactive current is changed twice in a fundamental cycle. This leads to the lower speed of changing the AC current during dynamic conditions. So it is necessary to find a single time instant to change the active and reactive current at the same time. Also, the worst scenario for changing the active and reactive current is not discussed. Moreover, the effect of low-order harmonics on the DC transient offset of the grid-tied converter for both low- and high-switching frequency were not discussed.